126 
ANIMAL PEDIGREES. 
June, 1891 . 
all the evidence we require. Many serious attempts have 
already been made to work out in detail this comparison 
between fossils and the developmental stages of living forms, 
and the results obtained are most promising. 
Following the lines laid down by his father, Alexander 
Agassiz has made a detailed comparison between the fossil 
series and the embryonic phases of recent forms in the case 
of the Ecliinoids or Sea Urchins, a group peculiarly well 
adapted for such an investigation, as the fossil representatives 
are extremely numerous and well preserved, and the existing 
members well known and comparatively few in number. 
Agassiz shows that the two records in this case agree 
remarkably closely; more especially in the independent 
evidence they give of the origin of the asymmetrical forms 
from more regular ancestors. 
The young Clypeastroid, for example, has an ovoid test, a 
small number of coronal plates, few and large primary 
tubercles and spines, simple straight ambulacral areas, and 
no petaloid ambulacra ; in fact, has none of the characteristic 
features of the adult Clypeastroid, while the characters it 
does possess are those of geologically older and preceding forms. 
So again, in the group of Echinidse, the members of the 
comparatively recent polyporous group, in which each ambu¬ 
lacral plate bears more than three pairs of ambulacral pores, 
commence their existence in the older and more primitive 
oligoporous condition, and become polyporous through fusion 
of originally distinct ambulacral plates. 
Agassiz gives many other examples, and, from a careful 
consideration of the entire group, arrives at the conclusion 
that “ comparing the embryonic development with the 
palaeontological one, we find a remarkable similarity and 
again, “ the comparison of the Echini which have appeared 
since the Lias with the young stages of growth of the prin¬ 
cipal families of recent Echini, shows a most striking coin¬ 
cidence, amounting almost to identity, between the successive 
fossil genera and the various stages of growth.” 
In this connection Agassiz make a suggestion of much 
interest. We are apt, he says, to assume, and perhaps 
rightly, that enormous periods of time have elapsed during 
the conversion of genus to genus, but the fact that these very 
changes can be repeated before our eyes in a few days’ 
or even hours’ time, during the development of the indi¬ 
vidual animal, may, perhaps, afford us a hint that such 
enormous periods are not really necessary in historical 
development, but that transformation of one form to a widely 
different one may, under favourable circumstances, be effected 
with considerable rapidity. 
